Disclosed is a method and a device for operating a synchronous machine having a stator to which three winding phases are assigned, and a rotor. Synchronous machines are used in the field of automobile engineering where they are used, for example, for steering systems of motor vehicles. Synchronous machines can be equipped with permanent magnets on the rotor. However, they can also be equipped with exciter windings in the rotor. Synchronous machines can be embodied as salient pole machines in which the rotor has a pole wheel with pronounced poles. However, the synchronous machine can also be embodied as a solid pole machine with a rotationally symmetrical rotor.
The publication “Control of Electrical Drives”, Leonhard, W., second edition, Berlin, Heidelberg, New York: Springer-Verlag 1996, pages 309 to 317 discloses a controller for a synchronous machine with permanent magnets. The synchronous machine is controlled by a field-oriented control loop. Here, the currents and voltages are transformed into a coordinate system which rotates with the rotor, i.e., a “d,q” coordinate system. The transformation is denoted as a Park transformation. Within this context, the controlled variables are the rotational speed and the rotational angle of the rotor.
In the d,q coordinate system, the d axis coincides with the orientation of the respective poles of the rotor, while the q axis is perpendicular to the d axis so that the d axis is also referred to as a real axis, and the q axis as a virtual axis. In the case of the salient pole machine, the flux mainly forms in the direct axis of the pole wheel due to the pronounced pole located at this position, and the flux therefore utilizes the d,q coordinate system particularly advantageous.
The publication “Field Oriented Control of Three-Phase AC motors”, Texas Instruments, BPRA 073, Texas Instruments Europe, 1998, discloses a field-oriented control loop for a synchronous machine. Here, a current sensor system is provided for sensing the conduction currents of the synchronous machine. The sensed conduction currents are transformed by means of a Clarke and Park transformation into the coordinate system which rotates with the rotor, the d,q coordinate system. The transformed d motor currents and q motor currents are then respectively fed to a logic operation point to form a control difference with corresponding setpoint values of the d,q motor currents, and this control difference is then respectively fed to a PI controller. The actuation signal of the respective PI controller is then a respective d or q voltage which is then subject to an inverse Park transformation and fed to a space vector pulse width modulator which acts on a three-phase power inverter in order to generate corresponding conduction currents for the synchronous machine.